Anthony Liguori wrote:
> Izik Eidus wrote:
>
>> Anthony Liguori wrote:
>>
>>> Izik Eidus wrote:
>>>
>>>> Izik Eidus wrote:
>>>>
>>>>> Anthony Liguori wrote:
>>>>>
>>>>>> Izik Eidus wrote:
>>>>>>
>>>>>>> @@ -1058,8 +1038,27 @@ struct page *gfn_to_page(struct kvm *kvm,
>>>>>>> gfn_t gfn)
>>>>>>>
>>>>>>> gfn = unalias_gfn(kvm, gfn);
>>>>>>> slot = __gfn_to_memslot(kvm, gfn);
>>>>>>> - if (!slot)
>>>>>>> + if (!slot) {
>>>>>>> + get_page(bad_page);
>>>>>>> return bad_page;
>>>>>>> + }
>>>>>>> + if (slot->user_alloc) {
>>>>>>> + struct page *page[1];
>>>>>>> + int npages;
>>>>>>> +
>>>>>>> + down_read(¤t->mm->mmap_sem);
>>>>>>> + npages = get_user_pages(current, current->mm,
>>>>>>> + slot->userspace_addr
>>>>>>> + + (gfn - slot->base_gfn) * PAGE_SIZE, 1,
>>>>>>> + 1, 0, page, NULL);
>>>>>>> + up_read(¤t->mm->mmap_sem);
>>>>>>> + if (npages != 1) {
>>>>>>> + get_page(bad_page);
>>>>>>> + return bad_page;
>>>>>>> + }
>>>>>>> + return page[0];
>>>>>>>
>>>>>>>
>>>>>> Wouldn't it be necessary to assign page[0] to slot->phys_mem[gfn -
>>>>>> slot->base_gfn]?
>>>>>>
>>>> sorry, it seems like i missunderstand you in the answer i gave you.
>>>> it wouldnt be necessary to assign page[0] to slot->phys_mem[gfn -
>>>> slot->base_gfn], beacuse phys_mem wont have any memory allocate by
>>>> this time.
>>>>
>>>> with this patch, we are not holding anymore (when using userspace
>>>> allocation) array of all the memory at phys_mem.
>>>> beacuse now that the pages are swappable, the physical address
>>>> pointed by the virtual address all the time change (for example when
>>>> swapping happn)
>>>> so no one promise us that slot->phys_mem[gfn - slot->base_gfn] will
>>>> really point to page holding the gfn page.
>>>>
>>>> so what we did, is throw away the phys_mem array (also nice beacuse
>>>> it waste less ram), and at runtime we are getting the pages by using
>>>> the virtual address
>>>> beacuse the reference of the page get increased, it promised us that
>>>> untill we release it point to the gfn (release it by doing put_page)
>>>>
>>>> hope i was more clear this time :)
>>>>
>>> Yes, that makes sense!
>>>
>>> I wonder if there's a more elegant way dealing with older
>>> userspaces. For instance, is there any reason why we can allocate a
>>> userspace memory region on behalf of userspace. That way swap would
>>> even work with older userspaces.
>>>
>> if we can do that, yes swap will work on older userspace.
>>
>
> I think it's just a matter of calling do_mmap() with the appropriate
> parameters. It looks likes there's some drivers call do_mmap() directly.
>
>
This will halve the maximum size of virtual machines on i386 since
userspace will also mmap() the memory, and the virtual address space is
restricted to 3GB.
--
Do not meddle in the internals of kernels, for they are subtle and quick to
panic.
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